1. Field of the Invention
The present invention relates to a method, system and program for filtering raster data and, in particular, performing different types of filtering operations on raster data.
2. Description of the Related Art
To print an image, a print engine processor, referred to herein as a raster image processor, converts the image in a page description language or vector graphics format to a bit mapped image indicating a value to print at each pixel of the image. Each bit representing a pixel that is “on” is converted to an electronic pulse. The electronic pulses generated from the raster pel data at which to deposit toner turns the laser beam on to positively charge the surface of a rotating drum, which is an organic photo-conducting cartridge (OPC), that has a coating capable of holding an electrostatic charge. The laser beam turns on and off to beam charges at pixel areas on a scan line across the drum that will ultimately represent the output image. After the laser beam charges all pels on the scan line indicated in the raster data, the drum rotates so the laser beam can place charges on the next scan line. The drum with the electrostatic positive charges then passes over negatively charged toner. The negatively charged toner is then attracted to the positive charged areas of the drum that form the image. The paper, which is negatively charged, passes over the roller drum and attracts the toner as the areas of the roller drum with the toner are positively charged to transfer the toner forming the image from the roller drum to the paper.
Many laser printers may filter the bit map images using a look-up table to alter the pulses generated for each pixel to accomplish a certain filtering result. For instance, filters can be used to provide an economy mode where toner is reduced, remove jagged edges, improve print quality enhancement or reduce the density of images. Typically, the laser printer will gather an area of data and replace either one or all the pulse values for the pixels based on the gathered area of pixel data matching a value in the look-up table. Such look-up tables modify the pixel output by altering the pulse normally used for an “on” pixel value with a pulse width modulator to shorten the pulse width to reduce the electric charge the laser beam places on the roller. Reducing the pulse width reduces the charged are for the pel on the roller and, hence, reduces the amount of toner attracted to the roller for that pel, thus reducing the amount of toner used to represent the pel.
Because different look-up tables may be used to perform different types of filtering, it is possible in prior art systems to change the look-up table being used to filter the rasterized bitmap data. However, in order to change the type of filter being applied, in prior art systems, the printer must halt operations, clear the paper path, skip a page, and then load the new look-up table type, e.g., print quality enhancement, toner reduction, etc., to perform a different type of filtering operation.
There is a need in the art for an improved technique for applying different filtering to rasterized bitmap data to modify the electromagnetic pulses for the bit map pixels that are “on” to accomplish the filtering goal of print quality enhancement, toner reduction, etc.